A stability assessment approach via reachable set calculation is presented to efficiently evaluate the dynamics of microgrids. Due to their low inertia, microgrids are sensitive to the uncertainties introduced by power-electronic-interfaced renewables and loads. Through the reachable set-based method, the bounds of all possible trajectories of a microgrid under a series of disturbances can be directly obtained, which makes repeated traditional time-domain simulations unnecessary. Moreover, a zonotope is used to better quantify these uncertainties and is integrated into the reachable sets calculation procedure. Extensive testing shows that reachable set calculations enable an efficient analysis of disturbances impacts on a microgrids dynamics, as well as offer a potent tool for evaluating how far the system is from its stability margins and what actions should be taken by system operators. These salient features make reachability analysis a powerful tool for planning, designing, monitoring and operating future microgrid systems.